Although a tan has long been considered a status symbol indicative of good health and the ability to secure sufficient leisure time to enjoy outdoor activities such as swimming, tennis, golf, skiing and the like, it has become very evident that excessive exposure of the human skin to sunlight is harmful.
It is well documented that human skin is sensitive to sunlight and artificial light containing radiation of wavelengths between about 290 nanometers (nm) and 400 nm. Ultraviolet radiation of wavelengths between about 290 nm and 320 nm (UV-B region) has been known to rapidly produce damaging effects on the skin including reddening or erythema, edema, blistering or other skin eruptions in more severe cases. Prolonged or chronic exposure to radiation in this wavelength range has been associated with serious skin conditions such as actinic keratoses and carcinomas. In recent years, concern has also been expressed regarding ultraviolet radiation of wavelengths above 320 nm (UV-A region) and the adverse effects of such radiation on human skin. The radiation between 320 and 400 nm also contributes to the premature aging of the skin. In addition, recent studies indicate that chronic sun exposure limits the immuno-response of the skin. There is also evidence that a tan will offer some protection against burning but is quite ineffectual against other types of solar damage.
Growing public awareness that the enjoyment of outdoor activities must go hand in hand with adequate sun protection has led to an unprecedented growth in the area of sunscreen products. A desirable sunscreen product should have the following attributes: protection in both the UV-A and UV-B ultraviolet radiation ranges; maintenance of coverage, i.e., waterproof and perspiration proof; application and use convenience, i.e., ease of application, invisibility, non-staining and non-greasy; and freedom from irritation as a result of its ingredients, in particular, its active sunscreen ingredients. Of recent interest in this area have been some concerns over the irritancy and sensitization problems that may occur in some individuals utilizing sunscreen products with high SPF values containing organic sunscreen agents.
The effectiveness of a sunscreen product is indicated by its sun protection factor (SPF). The sun protection factor is the ratio of the amount of exposure (dose) required to produce a minimal erythema reaction in protected skin to the amount required to produce the same reaction in unprotected skin. The absolute dose differs from person to person and is largely dependent on one's genetic predisposition and ethnic origin. If a person would normally require ten minute exposure to sunlight to develop a minimal erythema reaction, this person when using an SPF 15 sunscreen product should be able to tolerate up to 150 minutes of sunlight before developing a minimal erythema. Recent public awareness of the problems of exposure to sunlight has led to a demand for sunscreen products with high SPF values, i.e., at or above SPF 8.
Ease of application and cosmetic appeal, on the other hand, are important in formulating sunscreen compositions. These characteristics rely on subjective evaluations such as visual and tactile impression by the user. Consumer research studies indicate that a sunscreen formulation should rub in easily, leave the skin non-sticky and, above all, should be invisible on the skin after application. Sunscreen compositions containing organic sunscreen agents have been found, in some cases, to be irritating to the skin. Therefore, use has been made of inorganic sunscreen agents, such as titanium dioxide and zinc oxide.
For example, Japanese Patent Application No. 1981-161,881, describes cosmetics containing 0.1-40% of ultrafine divided titanium oxide with a particle size of 10-30 nanometers (nm) which has been rendered hydrophobic. It indicates that when hydrophobically treated titanium oxide with a particle size of 10-30 nm is blended into cosmetic base materials, it transmits visible light but reflects and scatters the harmful ultraviolet rays. It has been found that when these titanium dioxide compositions are utilized as a sunscreen agent in sunscreen compositions, it may result in the loss of one of the most desired properties of such compositions, i.e., invisibility.
U.S. Pat. No. 5,028,417, issued Jul. 2, 1991, describes sunscreen compositions containing microfine titanium dioxide. The particle size of the titanium dioxide is required to be less than 10 nm. It also states that other sunscreen agents can be utilized with the titanium dioxide.
U.S. Pat. No. 5,340,567, issued Aug. 23, 1994 describes a sunscreen composition comprising a synergistic combination of titanium dioxide having a particle size of less than about 35 nm and zinc oxide having a particle size of less than about 50 nm with titanium dioxide and zinc oxide being present at given ratios.
German Patent No. 3,642,794 (1987) describes a cosmetic composition for preventing sunburn which contains 1-25% zinc oxide of a particle size of 70-300 microns. It further indicates that the composition may also contain titanium dioxide of a particle size of 30-70 microns. This composition is undesirable due to its unaesthetic whiteness characteristics at high SPF levels.
U.S. Pat. No. 5,188,831, issued Feb. 23, 1993, describes sunscreen compositions wherein the sunscreen effect is obtained from a blend of oil-dispersible ultrafine titanium dioxide and water-dispersible titanium dioxide. However, the SPF level obtained is only of 10 with a total concentration of titanium dioxide of 5.0% w/w.
World Patent Application WO 90/06103, published Jun. 14, 1990, describes titanium dioxide sunscreens where the microfine titanium dioxide particles are coated with a phospholipid, either through the use of a powder mill or through the making of a dispersion in an oil phase containing the phospholipid with a high shear mixer. The phospholipid coated titanium dioxide is then incorporated into sunscreen compositions. A high efficiency is claimed: the data presented shows SPF values of up to 11 for a 3.75% titanium dioxide concentration and up to 25 for a 7.5% concentration of titanium dioxide. The use of high shear mixer or a powder mill is a complicated and energy intensive process.
EP 535972 A1, published Apr. 7, 1993 describes a method of preparing sunscreens in which a dispersion of zinc oxide and/or titanium dioxide particles in an oil is formed by milling.
EP 619999 A2, published Oct. 19, 1994 describes an aqueous dispersion of particulate metallic oxide of particle size less than 200 nm mixed with an emulsifier and an oil phase and also an organic hydrophobic sunscreen to form an o/w emulsion. The resulting sun protection composition has a higher SPF than would be expected if there was only an additive effect. However, the titanium dioxide alone at 4% yielded a SPF of only 7 to about 11.
EP 628303, published Oct. 19, 1994 describes a process for preparing a sunscreen composition. It consists of mixing sunscreen particles of metallic oxide less than 200 nm dispersed in an oil with one or more emulsifier and/or organic sunscreens. The resulting sunscreen composition is claimed to have a SPF value considerably higher than expected. The high SPF is only obtained when a metallic oxide is blended with an organic sunscreen. In fact, when no organic sunscreen is used, the SPF value is only about 7.
WO 93/11742 describes sunscreen compositions comprising titanium dioxide and iron oxide of particle size less than 200 nm preferably coated with a phospholipid.
An article published in DCI in September 1992 by Tioxide Specialties Ltd. describes various ways of incorporating oil or water dispersions of titanium dioxide in emulsions. However, no data is given on the resulting SPF values.
An article published in Cosmetics and Toiletries, Vol. 107, October 1992, describes various ways of formulating with a physical sunblock. The discussion focuses on using titanium dioxide in a dispersion or using an emulsifier which is also an effective dispersing agent for titanium dioxide. It states that SPF's far above 20 can be achieved. However, no examples are given, nor does the article mention the specific sunscreen components or their compositions.
A brochure published by the Tioxide Company on Mar. 15, 1994, discloses inorganic sunscreens of high SPF values obtained without the addition of any organic sunscreens. When measured, the SPF of the sunscreen compositions was indeed that described. However, when the titanium dioxide concentration was measured, it was at least twice what was claimed.
U.S. Pat. No. 5,498,406 describes sunscreen compositions in an oil-in-water emulsion containing both organic and inorganic sunscreens and comprising long chain (C.sub.25-45) alcohols for stabilization of the emulsion. This composition relies predominantly on the organic sunscreen actives. While the authors mention the use of stearic acid as a part of the oil in water composition, they teach against the use of stearic acid in stabilizing the titanium dioxide without C.sub.22-45 alcohols.
A sunscreen was marketed in the late 1980's under the trademark SUNDOWN.RTM., which contained both organic and inorganic sunscreen ingredients. In addition, it contained Velsan D8P3 and isostearic acid. However, it was not an efficient sunscreen, despite the addition of both organic and inorganic sunscreen ingredients.